1. Field of the Invention
The present invention relates to an inverter device.
2. Description of the Related Art
Switching regulators and inverter devices are used in order to supply high voltages to various devices, such as discharge tubes for large plasma displays and plasma generation apparatuses.
For example, atmospheric-pressure plasma is applied to various industrial products as a means for surface processing for, for example, modifying the surface or removing contaminants. When adhesion of, for example, a resin, printing, or coating is performed, pre-processing by atmospheric-pressure plasma makes it possible to increase the wettability.
Specifically, for example, when a print on which resin toner is printed by an electrophotographic image forming apparatus is to be coated with UV curing varnish, the wax components contained in the resin toner may repel the vanish on the printed resin toner.
The surface processing using atmospheric-pressure plasma increases the wettability and thus enables varnish coating, which increases the added values of prints. In order to generate atmospheric-pressure plasma, a high voltage is necessary and it is also necessary to efficiently apply a high voltage by using an inverter device and constantly supply the radical species thus generated to a load.
In general, while many inverter devices whose output power value is about a few watts are used, for plasma generation apparatuses, high voltage inverter devices whose output voltage is a few tens of kilovolts and whose power value is a few tens of watts are used.
In a general switching regulator (AC-DC or DC-DC converter), a switching element switches a direct current voltage to intermittently apply it to a primary excitation winding of a voltage transformer. An alternating current output generated at a secondary output winding is rectified and smoothed and a direct current voltage is output. In order to keep the output voltage at a constant voltage, the output voltage is detected, a feedback voltage is generated, and, according to the feedback voltage, pulse-width modulation (PWM) control for controlling the duty ratio (the on-time ratio in a period) of the switching element is performed.
The PWM control controls the output voltage to keep it constant by increasing the on-width of a switching pulse to compensate the output power shortage when the output voltage lowers, and by reducing the on-width to limit the excessive output power when the output voltage increases on the other hand.
However, because the output from the inverter device is an alternative current, it is difficult to control the peak value (peak voltage value) to keep it constant regardless whether it is a half wave or a full wave.
This is because the peak time comes at one point, and also because there is a control delay and, as the frequency at which the output voltage waveform is repeated increases, the effect of the delay becomes greater and accordingly the peak voltage excessively lowers or increases.
When the output is an alternating current and the switching frequency is high at a few tens of kilohertz and the peak voltage of the output is also high at a few tens of kilovolts, a problem of pressure resistance of an output voltage detection unit and parts occurs in addition to the above-described problem of control responsiveness. For this reason, in general, such a high voltage inverter device only controls the input supply voltage to keep it constant and does not control the output voltage value.
However, needless to say, to the device that uses the output voltage of the inverter device, it is preferable that the peak value of the output voltage does not fluctuate.
In order to meet such a demand, Japanese Laid-open Patent Publication No. 2013-31338 proposes that, in a high voltage inverter device whose output is an alternating current and its peak voltage is high at, for example, a few tens of kilowatts, the peak voltage is controlled to be kept constant.
The inverter device according to Japanese Laid-open Patent Publication No. 2013-31338 uses, as a monitor voltage, a voltage that is generated between the terminals of a switching element that switches the excitation current to a transformer or both ends of an excitation winding. A control signal is generated for controlling the period during which the switching element is on according to the peak value of the monitor voltage from the time at which the half wave of the monitor voltage in an off period of the switching element ends until just before the second peak wave of a resonance voltage appears. The on/off of the switching element is controlled in a way that the pulse width of a switching pulse of a square wave pulse signal having a constant frequency is modulated to change the ratio of the period during which the switching element is on according to the control signal and the switching pulse is output.
In this manner, to the output of an alternating current high voltage whose peak voltage is a few tens of kilovolts from an inverter device, it is possible to control the peak voltage to some extent.
The monitor voltage is, however, a voltage that is generated between both terminals of a switching device that switches the excitation current to a primary transformer or both ends of an excitation winding, and thus it is not possible to accurately detect a fluctuation in the output voltage due to, for example, a fluctuation at a load and perform feedback control.
Therefore, it is desirable to, in an inverter device that outputs an alternating current and its peak voltage is a few tens of kilovolts, enable fast feedback control by accurately detecting a fluctuation in the output voltage due to, for example, a fluctuation at a load, thereby keeping the peak value of the output voltage constant.